1. Field
This patent specification relates to electromagnetic measurements made in connection with boreholes. More particularly, this patent specification relates to methods and systems for correcting for or determining attenuation and/or phase due to a conductive casing of a borehole used in making electromagnetic measurements.
2. Background
Cross-well electromagnetic surveys have been used in the past to map the formation between two wells in oil-field environments. There are many papers in this area, among the earliest ones are modeling studies in 1995 by B. R. Spies and T. M. Habashy (see, B. R. Spies and T. M. Habashy, Sensitivity analysis of crosswell electromagnetics, Geophysics, Vol. 60, No. 3, P. 834-845 (1995)), by D. Alumbaugh and H. F. Morrison (see, David L. Alumbaugh and H. Frank Morrison, Theoretical and practical considerations for crosswell electromagnetic tomography assuming a cylindrical geometry, Geophysics, Vol. 60, No. 3, 1995, P. 846-870), and experimental studies by M. J. Wilt et al. (see, M. J. Wilt, D. L. Alumbaugh, H. F. Morrison, A. Becker, K. H. Lee and M. Deszcz-Pan, Crosswell electromagnetic tomography: system design considerations and field results, Geophysics, Vol. 60, No. 3, 1995, P. 871-885). More recently, regarding a new generation crosswell EM system, see Luis DePavia, Ping Zhang, David Alumbaugh, Cyrille Levesque, Hong Zhang and Richard Rosthal, Next generation cross-well EM imaging tool, SPE, 2008. Surface to borehole EM has also been considered, for example, to track water fronts in CO2 and water injection oil fields.
Among the above technologies, it is often the case where at least one tool string needs to be put into metallic cased wells for data collection within depths of interest. The inhomogeneities of well casing, both in thickness, diameters, and electromagnetic properties, make it challenging to remove these casing imprints on EM data in order to get high resolution inversion images. There are some known methods related to casing imprints removal. For example, see U.S. patent application Ser. No. 12/996,524 (Provisional US. Patent Application Ser. No. 61/075,913 filed on Jun. 26, 2008), and U.S. patent application Ser. No. 12/117,089 (U.S. patent application Ser. No. 12/117,089, filed May 8, 2008) hereinafter referred to as “the '089 application,” and incorporated herein by reference.
The casing correction described in the '089 application Patent Application involves using numerical modeling codes to calculate the casing attenuation and coil impedance to build up a look-up table, and then search for the corresponding metallic casing attenuation factor for the measured impedance at given frequencies.